Flakyness: fix webmidi/send-messages.html

Flakyness: fix webmidi/send-messages.html

This is using the new PermissionsHelper.setPermission() method that
prevents race conditions when trying to use testRunner.setPermission.

BUG=519008
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=201150

[mlamouri (slow - plz ping), 2015-08-25 20:08:25]

mlamouri@chromium.org changed reviewers:
+ avayvod@chromium.org

[mlamouri (slow - plz ping), 2015-08-25 20:08:32]

mlamouri@chromium.org changed reviewers:
+ lalitm@google.com

[whywhat, 2015-08-25 20:09:20]

lgtm

[mlamouri (slow - plz ping), 2015-08-25 20:34:23]

The CQ bit was checked by mlamouri@chromium.org

[commit-bot: I haz the power, 2015-08-25 20:34:36]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1314763003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1314763003/1

[commit-bot: I haz the power, 2015-08-25 20:51:47]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=201150